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wait but this should only work on locals / close networks shouldn't it ? i thought you still need some proxying in other cases (hence the turn) - i really need to study this again asap tough

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STUN gives back your public IP:port, TURN gives you assigned proxied IP:port.

You take that data and send to the peer over signaling connection, and they call you back on that IP:port. Most NAT implementations make and keep a temporary mapping between public port to private IP consistent[1] for few minutes, and not completely random per destination[2], so it usually works.

1: e.g. router.public.ip.example:23456 <-> 192.168.0.12:12345

2: e.g. if stun.l.google.com:12345 sent from port 23456 but if yourfriend.router.ip.example:12345 sent from port 45678

woaw thank you ; will definitely hop back on this topic now ; very much appreciate the answer

Yes. Unless the party generating the QR code first obtains its external IP address by other means, which would still require some kind of echo server. Even then, ignoring outdated approaches like UPnP, a commonly accessible host would be needed to establish signalling with e.g. NAT hole punching for anything but the most basic of setups.

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